The NTSC color television system presently employed in the United States utilizes a composite television signal including luminance and chrominance (color) signal components. The luminance components include contrast and brightness information, and the chrominance components are transmitted on a suppressed subcarrier wave which is phase modulated to represent hue and amplitude modulated to represent saturation of the color subcarrier in accordance with the ratio of the amplitude of the color subcarrier wave and the brightness information of the luminance component.
In addition to the luminance and chrominance components of the composite signal, there are included horizontal and vertical synchronizing components and a color synchronizing burst component transmitted at the end of each line of video information. The color synchronizing burst component transmitted at the end of each line of video information. The color synchronizing burst component includes a number of cycles of signal at the frequency of the subcarrier wave and is selected to have a standard phase relationship with respect to the phase of the modulated subcarrier wave. Thus, by phase and frequency synchronizing a reference oscillator in a television receiver to the burst component accurate reproduction of hue of the transmitted color theoretically should be effected.
The brightness and chrominance signal components are processed in different portions of the television receiver, and most color television receivers use a separate gain control circuit for the chrominance amplifier. Generally, the system for controlling the gain of the chrominance amplifier is an automatic chrominance control or ACC system which is responsive to the amplitude of the received color burst components of the composite signal. The color synchronizing burst component theoretically has an amplitude bearing a predetermined relationship to the amplitude of the modulated subcarrier, so that an automatic gain control system responsive to the burst amplitude theoretically may be used to cause the reproduction of the proper saturation of the transmitted color components by the receiver.
In actual practice, however, the burst component does not always have the theoretical correct phase and amplitude relationships to the transmitted color signals. As a consequence, in television receivers relying solely upon the burst component to establish the gain of the chrominance amplifier in an ACC loop, reproduction of a color image of improper saturation often results.
Other attempts have been made to cause the gain control of the chrominance amplifier to be effected by a peak detecting circuit responsive to the chrominance signal instead of or in addition to the burst responsive gain control circuit. This type of system theoretically is superior to an ACC system responsive solely to the burst components of the composite signal, particularly when this control is interrelated with the settings of contrast and brightness in the luminance channels. A problem with controlling the gain of the chrominance amplifier in response to a peak detection of the chrominance signal components is that for pictures which have a large background of heavily saturated colors, a peak detection system of this type applies substantial gain reduction to the chrominance amplifier undesirably desaturating the entire picture. Pictures of this type occur frequently enough that it is desirable to have an automatic chrominance control which responds to variations in the chrominance amplitude, but which ignores large areas of heavily saturated colors in the picture field.